1. Technical Field
The present invention relates generally to the field of wireless communication; and more particularly to a wireless communication system in which multiple connections are possible between a mobile unit and a plurality of base stations, connections between the mobile unit and the plurality of base stations are determined based upon the relative strength of pilot signals received by the mobile unit from the plurality of base stations.
2. Related Art
Wireless communication systems are well known in the Art. Examples of such wireless communication systems include cellular phone systems in which wireless communications are provided within a service area. In a typical wireless communication system a plurality of base stations or base transceiver stations each provide wireless coverage within a particular cell. Each base station may provide a plurality of sectors that form a respective cell. References made herein to base stations or cells also include reference to base station sectors. The base stations are coupled to a mobile switching center (MSC) which provides an interconnection between the base stations and the public switch telephone network. In combination, the MSC and these base stations manage ongoing communication within the service area and are commonly referred to as the "network."
Various protocols exist to manage communications within the service area. Such protocols include, for example, the Advanced Mobile Phone System (AMPS) standards, the Narrowband Advanced Mobile Phone Service (NAMPS) standards, the Global Standard for Mobility (GSM), the Code Division Multiple Access (CDMA) standard and the Time Division Multiple Access (TDMA) standard among others. While typical wireless communication systems may support only a single protocol within its service area, as systems progress, many systems provide support for multiple protocols within the service area.
Basic goals exist for operating mobile communication systems. Call clarity, call coverage, availability of carrier, maximum user density, and minimization of dropped calls are basic goals in operating the wireless communication system. As is known, depending upon the communication protocol supported by the communication system, reaching such goals must be accomplished in different fashions. For example, in a CDMA system, each base station can provide only a maximum power output within its respective cells or sectors. This maximum power output must be divided among the mobile units operating within the respective cells or sectors. Thus, as the number of mobile units operating within a cell or sector increases, the additional amount of power available for new mobile units decreases until a minimal level is reached and a maximum number of users has been reached for the cell or sector. Dividing the power in such a fashion sometimes results in dropped calls due to the signal strength going below a required threshold. Thus, it is desirable to only communicate with those mobile units within the base station's range as is required.
In CDMA systems, to minimize the dropped call probability and to improve call quality, mobile units typically communicate with more than one base station (cell) or more than one sector of a particular base station. Operations relating to the addition of a communication link between a mobile unit and a cell/sector is commonly referred to as a "hand-off". Hand-off operations also include dropping connections between the mobile unit and a cell/sector. When mobile units roam through a service area served by multiple base stations (cells and/or sectors), based upon continual evaluations by the mobile unit and communications with the network, hand-off operations occur frequently.
In prior systems operating according to the IS-95A CDMA standard, a determination of whether the mobile unit would seek connection to a particular cell/sector was made based upon the strength of a pilot signal received/measured from the cell/sector. When the strength of the pilot signal exceeded a threshold, the mobile unit notified the base station(s) with which it was already communicating by sending a pilot strength measurement message (PSMM). The PSMM was received by the network, and the network then made a determination as to whether the mobile unit should begin communication with the additional cell/sector. The determination was based in part on whether resources were available. If such was the case, a hand-off direction message (HDM) or an extended hand-off direction message (EHDM) was sent to the mobile unit. Upon receiving the HDM, the mobile unit added the new sector or cell to its active set and acknowledged such addition via a hand-off completion message (HCM).
However, if the network determined that the mobile unit should not add the new cell/sector to its active set, the network acknowledged the PSMM with a base station acknowledgment order (BSAO) but did not send the HDM. Such action prevented the mobile unit from again sending PSMMs for the particular candidate cell or sector until its pilot signal strength exceeded the weakest pilot signal strength of a cell or sector in the active set by the level T.sub.-- COMP. If such occurred, a second PSMM was sent for the cell or sector. The details of such operation are more specifically spelled out in the specifications for the IS-95A CDMA standard.
These techniques, however, had various shortcomings. The technique did not consider how many cells or sectors the mobile unit already had in its active set (those with which communication already was established). Thus, it was unknown whether adding an additional cell or sector to the active set would reduce the probability of a dropped call and increase call quality. However, by adding a cell or sector to the active set, its available power that could be delivered to other mobile units decreased, resulting in reduced network capacity.
Further, the thresholds used in determining whether to seek communication with another cell/sector were typically set in a fixed fashion for the duration of the call and did not consider the various other operating conditions faced by the mobile unit within the service area. In some cases, if the mobile unit was operating in a fringe area, it may be beneficial to add cells/sectors to the active set even if the pilot signals are relatively weak as compared to the fixed thresholds. Such additions often times decreased the probability of dropped calls and increased call quality.
An additional problem with such prior art technique relates to the construction of the mobile units. The mobile units typically include a rake receiver that could demodulate a maximum number of signal paths, typically three signal paths. As additional cells/sectors were added to the active set, to achieve a benefit from such addition, communications received from the additional cells/sectors had to be demodulated. However, because the mobile units could only demodulate a limited number of signal paths, the mobile units could often not demodulate all signals intended for it from the network. This problem increased system traffic and noise and reduced system capacity with no benefit to the intended mobile unit.
Thus, there exits a need in the art for a wireless communication system having improved hand-off performance, with such improved hand-off performance based upon the benefit of handoffs and implemented in a fashion to reduce dropped call probabilities, to reduce hardware resource requirements, to increase call quality and to increase network capacity.